Bone is formed by bone cells, collagen networks and crystals of hydroxyapatite (HA) on or within the collagen fibers. Decalcification refers to processes of removing HA from the collagen fibers, which is a technique used for processing bone specimen for pathologic diagnosis or producing surgical graft material. Decalcification can be performed by using acid or other agents. It is a well known cleaning process in industry to use certain cleaning agent in conjunction with ultrasonic vibration, since ultrasonic vibration can enhance the cleaning effect of the cleaning agent. CN 87100784 describes such a method using 1 to 1.7 MHz ultrasonic vibration, which may decalcify bones thin bone sheet (thickness<0.4 cm) within a few hours. However, its design may not be capable to decalcify large bone block.